Acoustic Camera
The idea is to build a camera capable of capturing images using sound as the illumination source rather than light. There are a number of ways to create images from sound, the most common of which usually involve phased arrays of some sort. This is how medical ultrasound and certain types of radar are often done, and the approach has many advantages. Another method is more similar to a light camera, except that the sound waves are focused like light waves by a simple type of lens, such as a pinhole or a zone plate. This type of camera would be extremely large, possibly filling a small room for even relatively high audible frequencies. It would require far more circuitry and have some other disadvantages, but it has a certain old-school retro cool factor about it. Questions to answer / Decisions to make
- Acoustic pinhole / zone plate camera?
- What frequency of audio to use?
- 10kHz might be a good target. It's still audible, but the wavelengths are small enough that the camera won't take up a whole building!
- Partly depends on whether we want to capture ambient sound or if we can illuminate the scene ourselves with sound sources
- How to capture all that data?
- Perhaps implement a bucket brigade circuit similar to a CCD
- Perhaps give each pixel an envelope detector so you don't have to sample each microphone at audio frequencies
- This would eliminate any possibility of doing further processing, such as using an FFT to separate the audio from each mic into different frequency components to display in different colors
- Mechanically sweep a few microphones to build up the image over time
- What frequency of audio to use?
- Acoustic phased array?
- What sort of array design? Grid? Spiral?
- What type of circuitry topology?
- Perhaps an array of boards, each board having enough circuitry to record 8 channels. Boards are all slaved to a single computer which does all the processing
Example implementation
- Frequency: 10kHz (wavelength 33mm / 1.3")
- Microphone imaging array: 32x32 = 1024 microphones!
- Number of zones: 5 (3 open zones including center pinhole)
- Pixel pitch (distance between each mic): 2*wavelength (~67mm / 2.6")
- Focal length of zone plate: 2m (~79")
- Actual distance required to focus on subject 10m away: 2.5m (98")
- Imaging array physical size: 2.1m (~84")
- Field of view: 56 degrees
If we could sample 16 channels at a time and sampled for 100 periods (10ms) with 5ms dead time between switching microphones, it would take about a half second to build up one frame. Current thinking:
A phased array approach is in some ways less cool, but is more practical in several ways (smaller, reduced circuitry requirements, higher frequency bandwidth, etc). A modular design could require relatively minimal circuitry development and fab:
- Custom sound capture PCB designed
- 8-channel ADC such as the ADS1178
- Preamp circuitry for 8 electret microphones
- Electret mics are extremely cheap and can be wired offboard to an array frame without much worry about interference
- Single SPI interface
- Single array frame holds all mics from different boards together in a known geometry
- Existing logic board (FPGA or possibly DSP) connects multiple sound capture boards together, combining synchronized SPI streams from each board
- FPGA is ideal because processing complexity is minimal but I/O is high, need a separate SPI bus for each board
- Logic stores sound onto SD card for later retrieval and processing
- PC loads data file containing all mic channels, performs simple delay-and-sum beamforming to construct an image or animation of the sound
- FFT could be used to extract frequencies present, encode them as pixel colors
